Pressure monitoring systems for patient care are most frequently used in fluid flush systems. A typical flush system includes a pressurized bag in which the pressure is maintained at approximately 300 mm Hg or 400 cms of water. The purpose of the bag is to provide a source of fluid to flush the line to the patient, either to fill the line with fluid or to flush blood back to the patient after blood fills the system during blood withdrawal. It is also used as a sorce of continuous flush to keep blood from forming clots over the end of an indwelling catheter connecting the tubing to the patients vascular system. The pressure in the bag must be greater than the patients blood pressure (about 100 mm Hg.) so that it can overcome the back pressure during intermittent flush. Continuous flush occurs via a restrictor mechanism as is well known. Whether the system is used in the continuous or the intermittent mode, the system is in series with the pressurized bag and the downstream components of the system. For continuous flush, the system consists of a restrictor designed to allow about 3 cc/hour of fluid to flow across an upstream-downstream pressure gradient of 200 mm Hg (300 minus the back pressure of a nominal 100). A restrictor this tight adds nearly no additional pressure to the system, and what little it does produce is a continuous bias accounted for during the process of zeroing the system as described more fully hereinafter.
The intermittent flush is simply a bypass valve that allows the full upstream pressure to bypass the restrictor and produce a rapid flush of the system. Typically, such valves consist of a mechanism which, when pressed or sqeezed, allows a valving mechanism to open. Thus, the fail-safe condition of the valve is a no-pass condition, a safety feature necessary to protect the downstream environment (patient) from potentialy harmfull pressures and flows.
Such a system also includes a transducer. The transducer is traditionally a "gage" type of pressure sensing device, meaning that it senses system pressure and uses atmospheric pressure as it's reference. This type of transducer has been used because it requires only one surface of the transducer to be exposed to the system environment. The other side of the transducer, in addition to being exposed to atmospheric pressure, interfaces with the electronic and mechanical components of the transducer. Because of the technical nature of the transducer, little emphasis has been placed on true differential sensing. Instead, there has developed a nearly universal acceptance of the use of unchanging atmospheric pressure as a default.
The transducer may be placed upstream or downstream from the flush device. If it is downstream, the transducer chamber has two ports. Fluid enters one port as it travels from the flush device to the transducer. It then travels through the transducer, purging it of air, and exits the second port, toward the patient. If the transducer is located upstream from the flush device, it is "dead ended" on one arm of a "T" arrangement, with the fluid from the reservoir bag entering the stem of the "T" and then either passing upstream to the transducer or downstream to the patient. The second port of the transducer is then opened only to allow air to purge the system during priming. Then it is closed.
At various locations in the system, there are located stopcocks (valves) which serve the function of either isolating regions or of venting ports when appropriate.
The configuration of the system is such that a first column of tubing extends from the transducer (or flush device) to the patient. As a result, there is a continuous column of fluid extending from the transducer to the patients' vascular system and from there to his heart.
The reservoir bag is pressurized to 300 mm Hg, a nominal setting. While the system is set for this pressure, it's function is not guaranteed even if the pressure is maintained at a constant level. This is so because the flush system is most dependent on the setting as it depends on the upstream pressure to provide the driving force necessary to maintain the 3 cc/hour flow through the restrictor, assuming the downstream pressure to be a mean of 100 mm Hg.
But, in practice the pressure is not maintained at a constant value because the means for maintaining the upstream pressure--an inflated bag-- is an inconstant source, requiring frequent reinflations, and the pressure gages are not sufficiently precise. Added consideration are that the accuracy of the resrictors is highly variable and the downstream pressures are rarely at the designated target value because blood pressure varies widely. Frequently, flush systems are used to measure the venous system pressures, normally at mean pressures of 10-20 mm Hg or less where imprecisions of such systems lead to excessive continuous flush.
The transducer for such a system is available in a variety of types. Specifically, pressure is sensed either by electrical or mechanical means. Electrical transducers also are of a variety of types including strain gages, silcon chip, conductance, inductance, reluctance, etc. Mechanical types include diaphragm-needle gages, fluid columns, Mercury manometers, etc. The transducer is always "differential" in that it measures the target pressure relative to a second pressure. If the second pressure is atmospheric pressure, it is termed a "gage". If the second pressure is a vacuum, it is an absolute pressure. If the second pressure is a pressure within a controlled chamber, the transducer is called "differential".
The transducer may or may not be able to measure liquids as well as gases or it may be able to accept liquids on only one side of it's pressure sensitive member, allowing only gas on the other side of the member. It is therefor either dry/dry, wet/dry or wet/wet.
In most medical pressure measuring situations, either dry/dry or wet/dry instrumentation is used, whether it is electrical or mechanical. Wet/wet capability imposes more complex issues. In electrical transducers, one surface of the pressure sensing element can face the environment to be measured, but the backside of the sensing element contains sensitive electronic components. Making the backside safe for exposure to liquids is a more complex and expensive procedure. Additionally, for disposable systems, or for nondisposable systems which must be cleaned, constructing an isolating chamber for the backside imposes cost considerations that render a true differential wet/wet type of transducer impractical. Purely mechanical differential gages of transducers suffer much the same problems. In any case, the tranducer has to be calibrated to establish a reference or "zero" value. Zeroing a fluid filled transducer traditionally means isolating the chamber on the sensing side of the transducer and filling the chamber with fluid (if the system is to measure fluid pressure) and often opening the transducer to atmospheric pressure. The electrical or mechanical signal produced by the pressure sensing member under these conditions is taken as "zero". When the chamber is then close to the atmosphere and opened to the measuring pressure, the system has nulled all the factors except the ones desired to be measured, and reads the pressure relative to atmospheric pressure.
If the the system to be measured is distant from the transducer, and contains fluid, there must be a continuous fluid path or pipe connecting to the system to be measured. Since the fluid has a density greater than air, which is the reference medium generating the reference pressure, the vertical height of the fluid column between the transducer and the site of measurement must be considered. The pressure measurement at the transducer is the pressure at the site of importance plus or minus the pressure generated by the fluid column that exists between the transducer and the site of measurement.
From a practical point of view, this system causes some problems. In medical applications, a fluid filled medium that must stay stagnant yet be directly coupled to a the patient's circulatory system imposes considerable risk of contamination. For this reason, regulatory authorities impose strict rules to ensure integrity of the system. One of these rules is that the system must be "closed". The zeroing procedure described above, while allowed because it is necessary, by definition violates the closed integrity of the system. Additionally, opening the system, and then flushing the chamber to flush out any air which may have collected, causes fluid to spill out of the port, most often onto the floor or onto the patients bed. Such practices certainly are undesirable at best.
In medical applications, the blood pressure that is taken as standard is the pressure at the central aorta at the point of exit from the heart. Venous pressure is taken as Central Venous Pressure (CVP), which is the pressure that exists in the right Atrium of the heart. Other physiological pressures are often taken at specific locations within the body, e.g. the brain, the pulmanary artery, the chest, the esophagus, the stomach, etc.
Two procedures must be followed to ensure that the transducer in each of these procedures, reads correctly. First, the zeroing procedure, described above, must be undertaken. This causes the transducer to read atmospheric pressure as zero. Second, a levelling procedure must be undertaken to ensure that the pressure read by the transducer corresponds exactly with the pressure at the site being measured, by physically eliminating any vertial columns. Simply speaking, this means that some method is employed to move the point of the transducer that was opened to air during the zeroing procedure to a vertical level exactly the same as the vertical height of the target site.
The methods employed for this second procedure are somewhat crude and basic. Most commonly, the practitioner "eyeballs" the system to a point that "looks right". If available, an actual levelling device is used. A somewhat more sophisticated approach is to take the tubing which goes to the patient and hold an exposed and open end of it at a point immediately adjacent to a point on the body closest to the target site. Such an approach, of course, exposes the system to contamination. Once the tubing is in the proper location, one may physically adjust the vertical height of the transducer until it reads zero, or one may "tell" the instrument that the pressure it then reads is to be called "zero" In any case, should the patients vertical height change or should the vertical position of the transducer change, this procedure must be repeated. In practice, this is one of the greatest sources of inaccuracy.
The problem with this arrangement is that, in normal operation, changes in the vertical position of the patient with respect to that of the pressure sensor appears to the sensor as a change in blood pressure. A mere four inch change in vertical height can signal a 10% change in blood pressure for a normal patient with a blood pressure of 100 centimetes (CMS) of water. For neonatal patients, where blood pressure normally is 40 to 50 CMS of water, that change in vertical height can signal a dramatic change in blood pressure calling for heroic efforts on the part of attending health care specialists which may be counterproductive or even fatal.